1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus and a communication method for performing mesh type communications using a CSMA (Carrie Sense Multiple Access) scheme.
2. Description of the Related Art
A terminal conforming to the CSMA scheme performs virtual carrier sense for a random period before transmitting a packet, and actually transmits a packet after recognizing that another terminal does not communicate with a base station. When another terminal is communicating with the base station, the packet is transmitted after the communication completes. At this time, every terminal fairly has a transmission right. In the scheme called virtual carrier sense, backoff control is performed after a channel becomes idle only for an IFS (Inter Frame Space) period.
In the backoff control, after the channel becomes idle, a random number is generated within a prescribed CW (Contention Window) range so that the random period is determined according to the random number. Then, the random number is determined as an initial value so that the initial value is decreased as time elapses. When the number becomes 0, the packet is actually transmitted. The IFS is defined in the wireless LAN standard IEEE 802.11, and is a period during which idle detection is performed before transmitting a packet. CW is a maximum value of the random number that can be taken for backoff, and is a parameter necessary for realizing user multiplication.
In IEEE 802.11, a minimum value CWmin and a maximum value CW max for the CW are defined. For backoff control in the initial transmission, the random number is calculated using CWmin so that the value of CW is doubled each time when retransmission is performed. By the way, CWmax is an upper limit value of CW. According to the backoff control using randomness, it becomes possible that a plurality of terminals perform communication by sharing a same channel. But, according to this scheme, there is a possibility that a plurality of terminals may transmit a packet at the same time. In such a case, packet collision occurs so that the packet cannot be received correctly. As a result, communication quality is deteriorated.
For handling this problem, priority control is performed in a communication scheme described in the following non-patent document 1.
[Non-patent document 1] IEEE Std 802.11e-2005, Amendment 8: Medium Access Control (MAC) Quality of Service Enhancements (IEEE 802.11e), 11, Nov. 2005
EDCA (Enhanced Distributed Channel Access) is defined in IEEE 802.11e. In EDCA, four types of priorities are prepared for packets, and, a transmission waiting time such as the IFS, CWmin and CWmax is set to be short for a packet having high priority so that a packet transmission right is provided for the packet preferentially. Accordingly, in an communication environment in which various applications such as voice and data are mixed, relative priority control can be performed.
In addition, in the IEEE 802.11e standard, a continuous transmission scheme called TXOP (Transmission Opportunity) is defined. For describing this scheme, a communication scheme of IEEE 802.11 is described based on the following non-patent document 2.
[Non-patent document 2] ANSI/IEEE std 802.11, Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, 1999
As shown in FIG. 1, this scheme uses a DCF (Distributed Coordination Function) scheme in which obtaining transmission opportunity (transmission right) based on backoff is necessary for transmission of each packet. In the figure, “D” indicates a packet sent from a node to another node, and “A” indicates an acknowledgement response (ACK, for example) in response to receiving a data packet. “DIFS” indicates a fixed period, determined in this standard, for which data packet transmission should be waited. “BO” indicates a variable period for which transmission is waited based on backoff, and the length of the period depends on a random number. As shown in the figure, when a communication node receives a data packet to be transferred, after waiting for a period defined by (DIFS+BO), the communication node sends one data packet. Also when the communication node receives a following data packet to be transferred, the communication node sends one data packet after waiting for the period defined by (DIFS+BO).
FIG. 2 is a diagram for explaining packet transmission using the continuous transmission scheme based on TXOP. In the diagram, “D”, “A” and “BO” indicate the same as those described in FIG. 1. “AIFS” is similar to “DIFS” in a point that it indicates a fixed period during which data packet transmission should be waited, but “AIFS” has a length that is variable according to priority of packet. “AIFS” is defined in IEEE 802.11e that is a standard of wireless LAN QoS.
In the TXOP scheme, as to initial packet transmission, one data packet is transmitted after waiting for a period defined as (AIFS+BO) in the same way as the case shown in FIG. 1. However, for transmitting a following second packet, backoff control is not performed. After a response packet “A” is received from a destination communication node, one data packet is transmitted after waiting for a fixed period. The fixed period is called SIFS (Short Inter Frame Space). Similarly, for following packets after the second packet, a data packet is transmitted one by one after elapse of the period of SIFS after receiving a response packet. Accordingly, packet transmission following the initial packet transmission is performed continuously.
By using the TXOP scheme, since overhead time before packet transmission is decreased, efficiency of wireless bandwidth is improved so that capacity improvement can be expected. For example, a number of terminals that perform VoIP (Void over Internet Protocol) communications simultaneously can be improved, throughput of data communication can be improved, or the like. Also, for example, in a case where a payload to be sent is extremely shorter than backoff before starting transmission like voice data in the VoIP communication, efficiency improvement of communication based on the TXOP scheme can be largely expected. In addition, in a wireless LAN mesh network, increase of overhead may become a serious problem since same data is transferred a plurality of times by APs (access points or base stations). But, by using TXOP, it can be expected that the problem can be effectively decreased. In addition, since the number of times of backoff control based on the random number is decreased, the possibility of packet collision is decreased. As a result, improvement of capacity can be expected.